1. Field of the Invention
This invention relates to a real image mode variable magnification finder which is suitable for use in a lens shutter camera or an electronic still camera in which a finder optical system is constructed to be independent of a photographing optical system, and in particular, to a finder which has a high magnification and a large angle of emergence and is most suitable for mounting in a compact camera.
2. Description of Related Art
In general, finders used in lens shutter cameras in which a finder optical system is constructed to be independent of a photo-graphing optical system are roughly divided into two classes: virtual image mode finders and real image mode finders. In the case where a finder has a variable magnification function and requires some degree of magnification at its telephoto position, the virtual image mode finder has the disadvantage that the diameter of a front lens must be enlarged, which constitutes an obstacle to compactness of the camera. The real image mode finder, in contrast with this, is such that its entrance pupil can be located on the object side of the finder optical system and thus the diameter of the front lens can be diminished. In this way, the real image mode finders are used nowadays in many lens shutter cameras having variable magnification functions
As lens shutter cameras have become more compact in recent years, a finder mounted in the camera must also be downsized. In this case, the problem arises that if the power of a magnifier is increased to downsize an ocular optical system, dust particles lying in the vicinity of a field frame are also seen in sharp focus. Specifically, the focal length of the objective optical system is reduced due to the compactness, and with this, the design of a finder having low magnification has been promoted. Consequently, although compactness of the camera has been accomplished, the problem is raised that the field of the finder becomes smaller and thus it is difficult to observe a subject.
As one of requirements for a finder which is easy to use, a large angle of view of emergence of the finder is needed. This is because a finder with a large angle of view of emergence enables a photographer to have immediately an actual feeling of seeing a wide observation image plane of the finder.
As another requirement, a high magnification of the finder is needed. In particular, it is important that the variable magnification finder has a high magnification at its wide-angle position.
A compact camera is usually designed so that the finder is set at the wide-angle position immediately after a power source is turned on or when it is off. Since the area of view is wide, the photographer first sees an observation area at the wide-angle position and then begins to photograph. That is, the observation area of the finder at the wide-angle position is a reference image plane that he can look out over the entire photographic area available when the magnification is changed. In this way, the photographer first observes the image plane at the wide-angle position to change the magnification or consider the composition of a picture. In a finder such that a variable magnification ratio is particularly high, for example, more than 2, when the composition of a picture at the telephoto position is considered at the wide-angle position, an object image assumed as the composition of a picture at the telephoto position is relatively small if the magnification at the wide-angle position is low. Moreover, since the area of the object image thus available is narrow, the composition of a picture is difficult, and such a finder is inconvenient for use.
In order to increase the angle of emergence and the magnification of the finder, the finder itself must be enlarged under conventional design techniques. In a large-sized finder, the power of each lens unit is weak and its fabrication is easy. Such a finder, however, can be used in a large-sized camera only.
For these reasons, a finder which equally supplies three requirements that the angle of emergence is large, the magnification is high, and the size is small has never been provided.
Also, a finder having a relatively large angle of emergence is disclosed, for example, in Japanese Patent Preliminary Publication No. Hei 6-51201. On the other hand, a finder having a high magnification at the wide-angle position is disclosed, for example, in Japanese Patent Preliminary Publication No. Sho 64-65520.
However, in the finder set forth in Hei 6-51201, the angle of emergence is large, but the finder magnification at the wide-angle position is as low as 0.35.times., and thus the object image within the finder field at the wide-angle position diminishes in size. Furthermore, the entire length of the finder is as long as 67 mm or more, thus compactness is not achieved. Prisms are constructed of materials with high refractive indices and thus are high in cost.
On the other hand, in the finder disclosed in Sho 64-65520, the finder magnification at the wide-angle position is as high as 0.564.times., but the entire optical system is extremely enlarged. For example, a finder shown in the third embodiment of this publication is such that, in view of only the ocular optical system, its length is more than 27 mm. Hence, when an attempt is made to mount this finder in a small-sized camera, a camera design is considerably restricted, which is unfavorable.